1. Field of the Invention
This invention relates generally to the field of geophysical prospecting. More particularly, the invention relates to the field of seismic data processing. Specifically, the invention is a method for attenuating noise in dual sensor seismic data.
2. Description of the Related Art
In the field of geophysical prospecting, the knowledge of the subsurface structure of the earth is useful for finding and extracting valuable mineral resources, such as oil and natural gas. A well-known tool of geophysical prospecting is a seismic survey. A seismic survey transmits acoustic waves emitted from appropriate energy sources into the earth and collects the reflected signals using an array of receivers. Then seismic data processing techniques are applied to the collected data to estimate the subsurface structure.
In a seismic survey, the seismic signal is generated by imparting an acoustic signal into the earth from on or near the earth's surface, which then travels downwardly into the earth's subsurface. The acoustic signal may also travel downwardly through a body of water in a marine seismic survey. Appropriate seismic energy sources used to impart the acoustic signal may include explosives or vibrators on land and air guns or marine vibrators in marine seismic surveys. When the acoustic signal encounters a seismic reflector, an interface between two subsurface strata having different acoustic impedances, a portion of the acoustic signal is reflected back to the surface, where the reflected energy is detected by a seismic receiver. Different types of seismic receivers detect and measure the amplitude of different physical aspects of the passing seismic waves.
Appropriate seismic receivers may include particle velocity detectors on land and water pressure detectors in water. Sometimes particle motion or particle acceleration detectors are used instead of particle velocity detectors. Both seismic sources and seismic receivers may be deployed individually or, more commonly, in arrays.
In intermediate zones between land and deep water, such as wet land or shallow water, both hydrophones and geophones are often used together, collocated in pairs, and positioned on the floor of the body of water. This procedure is commonly called an ocean bottom cable survey or a bay cable survey.
Dual-sensor ocean bottom seismic data often contain substantially more coherent noise than hydrophone-only seismic data. This coherent noise can be attributed to mud roll and spurious trapped or guided wave modes (sometimes referred to as torsion waves or spurious S waves). The geophone is far more sensitive to these types of coherent noise than the hydrophone. This fact has been used in some methods known in the art to attenuate noise from the geophone signal. There are, however, other types of noise contained in typical dual sensor seismic data. These types of noise include, but are not limited to, multiple reflections such as water column reverberation, transient noise, and effects due to differences in receiver coupling. There are a number of methods known in the art to attenuate such noise, using the differences between the hydrophone and geophone signals.
For example, Dragoset, Jr., W. H., in U.S. Pat. No. 5,365,492, “Method for Reverberation Suppression”, issued Nov. 15, 1994, discloses a method for suppressing water-column reverberations in seismic data from dual sensor ocean bottom cable surveys. Pressure and velocity signals with embedded noise are detected concurrently from co-located dual sensors. The pressure signal is adaptively filtered and subtracted from the velocity signal to isolate a noise signature. The noise signature is added back to the velocity signal with opposite polarity to give a noise-free velocity signal. The noise-free velocity signal is multiplied by a scale factor and added to the pressure signal. The sum is auto-correlated and a function, referred to as a varimax function, is computed for the auto-correlated sum. The scale factor is incremented and the process iterated until the varimax function approaches unity. This yields the optimum scale factor for suppressing water-column reverberations.
Dragoset, Jr., W. H. and Chambers, R. E., in U.S. Pat. No. 5,442,591, “Method for Adaptively Suppressing Noise Transients in Summed Co-Sensor Seismic Recordings”, issued Aug. 15, 1995, disclose a method for attenuating singular transient noise such as caused by biological sources, such as fish, in seismic data from dual sensor surveys. Pressure and velocity signals with embedded noise are detected concurrently from co-located dual sensors. The adaptive method of Dragoset's '492 patent, discussed above, is applied to scale the velocity signals to the amplitude of the pressure signals. The pressure and velocity signals are separately grouped to form common receiver gathers. The ratio R1 of the amplitudes of the velocity and pressure common receiver gathers is determined. In weighting zone windows, an array of ratios R2i of the amplitudes of the pressure and velocity signals is determined. The ratios R1 and R2i are multiplied to give an equalization operator mi. The pressure and velocity signals are combined with the equalization operator in each weighting zone window to yield a transient noise-free time-scale datum.
Chambers, R. E., Sifton, G. A., and Paffenholz, J, in U.S. Pat. No. 5,572,483, “Method of Reducing Noise in Seismic Signals by Adaptive Filtering of a Noise Reference”, issued Nov. 5, 1996, disclose a method for attenuating noise in seismic data represented as seismic traces. The suspected noise is located in a set of the seismic traces. Corresponding portions of the traces containing the noise are time-aligned and then stacked to form a stacked noise trace. The stacked noise trace is replicated at each corresponding trace position in the selected set of traces. Then the time-alignment procedure is reversed to generate noise signature traces at the original time positions. Filters are generated that minimize the difference between the noise signature traces and the original seismic traces. The noise signature traces are filtered and then subtracted from the seismic traces. The steps of generating filters, filtering and subtracting use linear adaptive filtering techniques.
Rigsby, T. B. and Sanders, J. I., in U.S. Pat. No. 5,621,699, “Apparatus and Method of Calibrating Vertical Particle Velocity Detector and Pressure Detector in a Sea-Floor Cable with In-Situ Passive Monitoring”, issued Apr. 15, 1997, disclose a method for suppressing water-column reverberations in seismic data from dual sensor surveys. Geophone and hydrophone signals with embedded noise are detected concurrently from co-located dual sensors. Geophone and hydrophone noise signals that represent ambient noise for the geophone and hydrophone, respectively, are obtained. A normalized relative noise signal of the difference between the geophone and hydrophone noise signals is determined. The relative amplitudes of the geophone and hydrophone signals are scaled by a function dependent upon the normalized relative noise signal. Then the scaled signals are linearly combined.
Starr, J. G., a co-inventor of the present invention, in U.S. Pat. No. 5,754,492, “Method of Reverberation Removal from Seismic Data and Removal of Dual Sensor Coupling Errors”, issued May 19, 1998, and in U.S. Pat. No. 5,825,716, “Method of Reverberation Removal from Seismic Data and Removal of Dual Sensor Coupling Errors”, issued Oct. 20, 1998, discloses a method for suppressing water-column reverberations in seismic data from dual sensor surveys. The '716 patent is a division of '492 patent. An upgoing and a downgoing wavefield is determined in the seismic data. The downgoing wavefield is multiplied by the free surface reflection coefficient and then added to the upgoing wavefield.
Starr's '492 patent and '716 patent also disclose a method for attenuating the effects of receiver coupling in seismic data from dual sensor arrays. A reverberation response period is first determined. A first cross-equalization filter is constructed as a function of the reverberation response period and a second cross-equalization filter is constructed as a function of the seismic data. An inverse coupling filter is derived as a function of the first and second filters. The inverse coupling filter is then applied to the data.
Geiser, J, Barr, F., and Paffenholz, J., in their publication “Vertical Component Coupling of OBC-Data”, EAGE 64th Conference & Exhibition, Florence, Italy, 27-30 May, 2002, disclose a method for attenuating vertical coupling effects in seismic data from three-component dual sensor ocean bottom cable surveys. Assuming that the hydrophone is perfectly coupled to its fluid medium, its pressure signal is used as a reference signal for the vertical component of the velocity signal from a geophone. A least squares relation between the pressure and vertical component signals is minimized to yield a correction factor for the vertical component of the velocity signal. The Geiser, et al. reference does not discuss how their method would apply, if at all, to one-component dual sensor seismic data.